1. Field of the Invention The present invention relates generally to an electro-optic high voltage sensor for sensing and/or measuring an E-field produced by an energized conductor. More particularly, it concerns an electro-optic voltage sensor which utilized the Pockels electro-optic effect to measure voltage.
2. Background Art
High-accuracy measurement of high voltage has traditionally been accomplished using iron-core ferro-magnetic potential transformers. These devices have substantially limited dynamic range, bandwidth, linearity, and electrical isolation. During electrical fault conditions these transformers can conduct dangerous levels of fault energy to downstream instrumentation and personnel, posing an additional liability.
A variety of optic sensors for measuring voltage have been developed in attempts to offer the power industry an alternative to the conventional transformer technology. Generally, these voltage sensor systems require that direct electrical contact be made with the energized conductor. This contact is made necessary by the use of a voltage divider which is utilized to connect the sensing element with the energized conductor on which a measurement is to be made. Direct electrical contact with the conductor may alter or interrupt the operation of the power system by presenting a burden or load.
In addition to the disadvantages associated with direct electrical contact with the energized conductor, prior art voltage sensor systems are typically bulky, particularly in extremely high voltage applications. This is true because the size of the voltage divider required is proportional to the voltage being measured. The size of such systems can make them difficult and expensive to install and house in substations.
Many prior art sensors are based upon the electrostrictive principle which utilize interferometric modulation principles. Unfortunately, interferometric modulation is extremely temperature sensitive. This temperature sensitivity requires controlled conditions in order to obtain accurate voltage measurements. The requirement of controlled conditions limits the usefulness of such systems and makes them unsuited for outdoor or uncontrolled applications. In addition, interferometric modulation requires a highly coherent source of electromagnetic radiation, which is relatively expensive.
Open-air E-field based sensors have also been developed, but lack accuracy when used for measuring voltage because the open-air E-field used varies with many noisy parameters including ambient dielectric constant, adjacent conductor voltages, moving conductive structures such as passing vehicles, and other electromagnetic noise contributions.
Systems which utilize mechanical modulation of the optical reflection within an optic fiber have also been developed. Among other drawbacks, the reliance of such systems on moving parts is a significant deterrent to widespread use.
U.S. Pat. No. 5,892,357, issued Apr. 6, 1999, and assigned to the same assignee of the present invention, discloses an electro-optic voltage sensor which may be disposed in an E-field between an energized conductor and a grounded conductor without contacting the energized conductor. The electro-optic voltage sensor utilizes a Pockles crystal or transducer which is sensitive to the E-field and induces a differential phase shift on a beam of electro-magnetic radiation traveling through the sensor in response to the E-field. Although the electro-optic voltage sensor solves many of the problems with the prior art, it still has some drawbacks. For example, the electro-optic voltage sensor disclosed in the above mentioned co-pending application utilizes a beam splitter to separate orthogonal polarization components of the electro-magnetic radiation. The beam splitter directs one component out of the sensor in one direction, for example along a longitudinal axis of the sensor, and directs the other component out a different direction, perpendicular to the longitudinal axis of the sensor. Therefore, either both components exit the sensor from different sides, making the sensor difficult to locate between the conductor and grounded conductor, or an additional reflector is required to direct the other component so both components exit the same side, making the sensor large.
It would therefore be an advantage in the art to provide a system which does not require direct electrical contact with the energized conductor, is compact, operates in a variety of temperatures and conditions, is reliable, and is cost effective.
It is therefore an object of the present invention to provide an electro-optic voltage sensor system which does not require contact with a conductor.
It is a flier object of the present invention to provide such an electro-optic voltage sensor system which is capable of use in a variety of environmental conditions.
It is a still further object of the present invention to provide such an electro-optic voltage sensor system which can be employed to accurately measure high voltages without use of dedicated voltage division hardware.
It is an additional object of the present invention to provide such an electro-optic voltage sensor system which minimizes the electronics needed for implementation.
It is a further object of the present invention to provide a sensor system capable of being integrated with existing types of high voltage power transmission and distribution equipment so as to reduce or eliminate the need for large stand-alone voltage measurement apparatus.
It is yet another object of the present invention to provide a sensor system capable of being integrated with existing types of power transmission and distribution equipment.
It is yet another object of the present invention to provide a sensor system with a sensor that is of small size.
While the present invention is described in terms of a sensor system, it is to be understood that the subject apparatus and method may be used in any field of electrical or optical application. Those having ordinary skill in the field of this invention will appreciate the advantages of the invention, and its application to a wide variety of electrical uses.
The above objects and others not specifically recited are realized in a specific illustrative embodiment of an electro-optical voltage sensor device and system whereby one may measure the voltage difference (or electrical potential difference) between objects or positions. Voltage is a function of the electric field hereinafter xe2x80x9celectric fieldxe2x80x9d shall be indicated xe2x80x9cE-fieldxe2x80x9d) and the geometries, compositions and distances of the conductive and insulating matter. Where, as in the present invention, the effects of an E-field can be observed, a voltage measurement can be calculated.
The sensor device may be utilized to sense or measure an E-field using a source beam of electromagnetic radiation. The sensor device comprises a sensor body disposed in the E-field. The sensor has an input for receiving the source beam into the sensor body. The sensor body also has first and second outputs.
A polarization beam displacer is disposed in the sensor body and is optically coupled to the input. The polarization beam displacer separates the source beam into a first beam having substantially a first linear polarization orientation and a second beam having substantially a second linear polarization orientation. The polarization beam displacer also directs the first beam along a first path and the second beam along a different second path. The second beam may be discarded.
A wave plate is disposed in the sensor body and is optically coupled to the polarization beam displacer for rotating the first polarization of the first beam to a rotated polarization with major and minor axes.
A transducer is disposed in the sensor body and is optically coupled to the wave plate. The transducer induces a differential phase shift on the major and minor axes of the rotated polarization in response to the E-field when the transducer is exposed to the E-field.
A reflecting prism is disposed in the sensor body and is optically coupled to the transducer. The prism redirects the first beam back through at least the polarization beam displacer means. The prism may also reflect the first beam back through the transducer and wave plate. The reflecting prism may also convert the rotated polarization of the first beam to circular or elliptical polarization.
The transducer may further induce a differential phase shift on the major and minor axes of the circular or elliptical polarization of the first beam as the first beam passes back therethrough. The wave plate rotates the major and minor axes of the circular or elliptical polarization of the first beam.
As the first beam passes back through the polarization beam displacer, the polarization beam displacer separates the first beam into a third beam representing the major axis of the first beam and a fourth beam representing the minor axis of the first beam. The polarization beam displacer also directs the third beam along a third direction towards the first output and the fourth beam along a different fourth direction towards the second output.
The invention may also comprise a graded index lens disposed in the sensor body between the input and the polarization beam transducer. The lens collimates the beam of electro-magnetic radiation. Other lenses may also be used to collimate and/or collect the third and fourth beams.
The invention may also comprise graded index lenses disposed in the sensor body at the first and second outputs. The lenses collect and focus the third and fourth beams.
The invention may also employ a transmitter, a detector, and a signal processor. The transmitter produces a beam of electro-magnetic radiation which is routed into the sensor device. Although this electromagnetic radiation used in the present invention can comprise any wavelengths beyond the visible spectrum, the term xe2x80x9clightxe2x80x9d, xe2x80x9cbeamxe2x80x9d, and/or xe2x80x9csignalxe2x80x9d may be used hereinafter to denote electro-magnetic radiation for the purpose of brevity.
The first beam undergoes an electro-optic effect when the sensor is placed into the E-field, and is observable as a phase differential shift of the major and minor axes of the elliptical polarization. The planes of propagation are the object of the differential phase shift. The differential phase shift causes a corresponding change in the beam""s polarization. The polarization change is in turn converted into a set of amplitude modulated (AM) signals of opposing polarity that are transmitted out of the sensor. The detector converts the set of optical AM signals into electrical signals from which the voltage is determined by the signal processor.
The sensor processes the beam by splitting the beam in accordance with the components of the orthogonal polarization planes into at least two AM signals. These AM signals are then processed in an analog circuit, a digital signal processor, or both. The AM signals may be converted to digital signals, fed into a digital signal processor and mathematically processed into a signal proportional to the voltage which produced the E-field. In addition, the AM signals may be optically processed. Alternatively, the output of the analog circuit are a sinusoidal waveform representing the frequency and peak-to-peak voltage and an RMS voltage. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or maybe learned by the practice of the invention without undue experimentation. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.